The present invention relates to a mobile communication system and a wireless base station control apparatus, and particularly to a mobile communication system and a wireless base station control apparatus for managing an adjacent relation of wireless base stations in a mobile communication system where plural wireless terminals simultaneously communicate with one sector and one wireless terminal can receive signals of plural sectors and/or in a mobile communication system where communication is performed by the code division multiple access (CDMA) system in each sector of each base station.
The basis of cellular communication is that a wireless terminal, such as, for example, an in-vehicle telephone or a cellular phone, finds a radio wave from a sector of a nearby base station and exchanges a radio wave between the wireless terminal and the base station. A communicable area in which a radio wave from one of antennas provided in a base station can reach is called a “sectors”. The base station includes one or plural sectors. The cellular communication service area is constructed by arranging the sectors without a gap. A multi-sector structure in which base stations are installed at intervals of a specific distance is adopted, and plural sectors provided by the respective base stations are disposed continuously (so as not to become thin), so that the wireless terminal can communicate from anywhere, and a wide service area is constructed.
Besides, in the CDMA communication network, a soft handover technique is known in which when a wireless terminal changes a base station with which communication is made, a composition of signals by communication with plural base stations or selection of a communication channel is performed, and the communication channel is changed without instantaneous interruption (for example, see “3G TR25.832 V4.0.0”, published by 3GPP, March 2001, Chapter 5.2.1). In order to perform this soft handover, it is necessary for a wireless terminal to previously receive a control signal from a sector of a base station (adjacent base station) adjacent thereto. In order to bring the soft handover into an operable state, the wireless terminal searches a radio wave of a different sector by sequentially using an offset value other than a pilot signal offset value used in the sector under communication, and attempts to capture the radio wave from the different sector.
On the other hand, the base station manages control information of the sector and periodically transmits (broadcasts) the control information. This broadcast message includes radio wave information of a peripheral sector called neighbor sector information (see, for example, “3GPP2: C. S0024”, published by 3GPP2, October 2002). The wireless terminal can efficiently capture the radio wave of the peripheral sector based on the neighbor sector information. By this, it becomes possible to effectively perform the soft handover.
For example, in the case where the “neighbor sector information” is set in the control information received from the sector under communication, the radio terminal attempts to capture the radio wave in accordance with a frequency and a pilot signal offset indicated in the “neighbor sector information”, so that it becomes possible to efficiently capture the radio wave from the neighbor sector. Besides, the wireless terminal simultaneously captures radio waves from plural base stations, compares reception qualities, and performs the handoff to one with a higher quality.
In the cdma system, in order to effectively perform the soft handover and to provide a high quality cellular communication service, it is appropriate that a neighbor sector is correctly determined in each sector, information of the determined neighbor sector is set and registered as management information of the base station, and a state is produced in which a broadcast message transmits the correct neighbor sector information. Besides, in the cellular communication system, also after the service is started, in order to eliminate a shadow area and to deal with an increase in traffic, the rearrangement of wireless base stations and the additional installation of a base station are frequently performed.
When the addition and change of base stations as stated above is performed, there is a case where the adjacent relation of the base stations is changed. In order that the wireless terminal efficiently continues the soft handover also after the adjacent base station is changed, the adjacent relation after the change is correctly grasped, and if necessary, it is necessary to change the notification (broadcast) message from the base station.
Besides, even in the case where there is no change in the arrangement of base stations, there is a case where a transmission environment is changed by the change of an environment around the base station due to the construction of a building or the like. For example, there is also a case where the soft handover to a certain neighbor sector becomes impossible by the change of the transmission environment. In such a case, for example, the information of the sector must be deleted from the broadcast message.
As a method of specifying sectors having an adjacent relation, it is conceivable to calculate the adjacent relation based on a distance between the base stations. However, even in sectors of base stations close to each other in distance, there is a case where the radio wave from the nearby sector does not reach by the influence of geographic features or a building, or there is a case where the radio wave is actually limited in a closed space as in an underground market or a base station installed in a building, and there is a case where the soft handover can not be performed. Thus, in order to strictly determine the adjacent relation of sectors, it becomes necessary to grasp the radio wave environment by a traveling test using a radio wave measuring vehicle, a transmission simulation or the like. However, it takes long time and high cost to measure the radio wave. Further, the measurement is required each time a base station is added or the arrangement is changed. Besides, the simulation also requires both time and cost, and since an actual environment can not be completely reproduced, a correct result is not necessarily obtained.